Low-frequency seam timer



May 23, 1950 J. R. PARSONS ET AL LOW-FREQUENCY SEAM TIMER 5 Sheets-Sheet 1 Filed Sept. 30, 1948 wow 5..

NON mom m} INVENTORS John R. Parsons 8| $dward C. Huriwig.

ATTORNEY I m8 I MMN\ H m 5 5 E; m, U EB 3553 32 5 y 3, 1950 J. R. PARSONS ETAL LOWFREQUENCY SEAM TIMER 5 Sheets-Sheet 2 Filed Sept. 30, 1948 INVENTORS John R. Persons 8 BY Edward C.Horiwig.

ATTORNEY y 1950 J. R. PARSONS ETAL LOW-FREQUENCY SEAM TIMER 5 Sheets-Sheet 5 Filed Sept. 30, 1948 May 23, 1950 J. R. PARSONS ETAL 2,508,467

' LOW-FREQUENCY SEAM TIMER Filed Sept. 30, 1948 5 Sheets-Sheet 4 eel 393 Weid lnter ml Timer Frequency Determining Circuits WITNESSES: INVENTORS Firing Control John R. Persons 8 Circuii ggiword C. Huriwig. gfifim ATTORNEY May 23, 1950 Filed Sept. 30, 1948 I ml J. R. PARSONS EIAL 2,508,467

LOW-FREQUENCY SEAM TIMER 5 Sheets+$heet 5 L a T.

WITNESSES: T lNVENToRs John R. Persons 8 Mm Edward O. Horiwig.

11 BY 7 I fii M ATTORNEY Patented May 23, 1950 2,508,467 LOW-FREQUENCY SEAM rnvmn John R. Parsons and Edward C. Hartwig, Buffalo, N. Y., a'ssignors to Westinghouse Electricj'Cor- 'poration, East Pittsburgh, Pa.', a corporation of Pennsylvania Application samba so, 194s,ser a n qsaios Our invention relates to electric discharge apparatus and it has particular relation to resistance welding apparatus.

- The subject matter of this application is the outgrowth of a program under which apparatus for supplying current to weld material at a frequency substantially lower than commercial frequency was developed. The following applications are also an outgrowth of this program and relate closely to the present application:

I An application for Welding apparatus, Serial No. 18,699, filed April 2, 1948, to-John R. Parsons; an application for Electronic sequence timer, Serial No. 47,812, filed September 4, 1948, to Edward C. Hartwig; an application for Frequency control system, Serial No. 52,103, filed September 30, 1948, to John R. Parsons; and an application for Low frequency control for welding, Serial No. 52,104, filed September 30, 1948, to John R. Parsons and Clarence B. Stadum.

In the second of the above listed applications, a timer is disclosed. While the apparatus disclosed in the other applications has general purposes, it is primarily conceived for producing spot welds of predetermined duration by transmitting alternating current of a frequency substantially lower than commercial frequency through the material to be welded. The welding systems disclosed in the latter applications embody certain of the features of the timer.

The spot welding apparatus disclosed in the above listed applications includes a system for converting commercial frequency alternating current to the lower frequency. Each of the spot welding operations performed withthis apparatus is an independent operation involving several steps. The electrodes are first engaged with the material to be welded during a so-called Squeeze interval; next the welding current is transmitted through the material during the "weld interval; thereafter the electrodes are held in engagement with the material until it cools during a so-called hold interval; finally the electrodes are disengaged from the material and are reset for a second operation during a so-called off interval. To time these steps, a sequence timer is also included in the spot welding apparatus. The timer is closely tied up with the converting system and the welding mechanisms.

Since the low frequency spot welding apparatus was developed, a demand has arisen for low frequency seam welding apparatus. A seam welding system differs radically from a spot welding system. In producing a seam weld, the several steps of a spot welding operation are not 5 Claims." (01. 3154250) involved. The welding electrodes are injthe form of rollers, .between which the material is moved. Once the electrodes are engaged with the material to be welded, they are maintained in engagement and the material moves continuously between them. The welding current is transmitted through the rollers and the material as a series of pulses. The. welding current is turned on for a predetermined on interval and is maintained off Curing-a second off interval. In a low frequency seam welder each of the intervals is equal to a periodor more of the low, frequency derived from commercial buses through a connector. The timer for a seam welder like the timer of Spot welder must necessarily be close 1y tied up with the frequency converter but the relationship for the former is different than for the latter.

During a seam welding operation, the material may move. at a relatively high speed between the electrodes. If the current transmitted during this operation becomes excessively high, a substantial quantity of the material treated may be burned before the improper operation is discovered and thedrive for the material stopped. Because of excessively high welding currents, the material to be welded may also adhere to the welding rollers. Under such circumstances the latter must be replaced frequently. If the welding current is too lowv the material may not be fused at the welds. In a plant where material for automobiles or aircraft is being welded such eventualities are serious catastrophes, particularly in these days of material shortages. Ac--' cordingly, in the development of a seam welder of the low frequency type, precautions must be taken to minimize the probability that these even: tualities will occur. The timing must be accurate and such phenomena as saturation of the welding must be precluded.

It is, accordingly, an object of our invention to provldea seam welder which shall operate with the welding current at a substantially lower frequency than commercial frequency.

Another objectof our invention is to provide apparatus for supplying current of a substantial- 1y lower than commercial frequency at discrete intervals to scam weld a material.

.A further object of our invention is to provide apparatus for seam welding with alternating current' .of, a substantially lower than commercial frequency without saturating the welding transformer- I An ancillary object of our invention is to provide a novel timer.

Another ancillary object of our invention is to provide a. novel electronic timer particularly adapted to cooperate with a frequency converter to produce alternating current pulsations of a substantially lower than commercial frequency.

In accordance with. our invention. We. provide a seam welder in: which the welding. current is derived from a frequency converting system controlled from a frequency determining circuitv which is in turn controlled from a weld interval timing circuit. The timing circuit-turns. theefl quency determining circuit on and off during, accurately timed intervals. During the; on linlatter actuates the converter to supply current to the welding transformer repeatedly first in one di rection for a predetermined jnu-ntberofperiods of the commercial supply and then in the opposite direction for an equal number of periods of the commercial supply. current at a substantially lower frequency than thecommercial frequency is thus supplied for welding, During the.off'intervals, the'frequency determining circuit and 'the converter are quiescent. The on-off timing circuit is so connected to the frequency determiningcircuit as to assure that the welding current, will flow during an equal number of positive and" negative half periods of the low. frequency;

Thenovel features that we consider characteristic, of our inventionv'are set forth with particularity in the appe ded claims. The invention During. the on interval, 1?.

tervals of the frequency determining .circuit tha former 1 is equal to the duration and magnitude of the current of the opposite polarity. A timing circuit 55 is provided for turning the frequency determining circuit on and ofi.

In the control circuit of each of the firing thyratrons 39 to 49, a network 51 for impressing a biasing potential is provided. This biasing potential may be counteracted by a potential which is, derived from a transformer coupled to the anode circuit of a corresponding thyratron 59, BI, :63;;; '65.,,61.amd 69 of the firing control circuit 53. These thyratrons 59 to 69 may be subdivided into .Iwvo'groups of three; each. One of the groups 59, 637311311161 controls. the firing thyratrons 39, 43 and 41,,1aespecitively. associated with the ignitrons 21, 3! and 35, respectively, which conduct current of -one polarity'and the other 5|, 65 and 69 with the firing thyratrons 4I 45 and 49 associated with the ignitrons 29, 33 and, 31 which conduct current of the opposite polarity.

The anode circuits ofthe thyratrons 59 and-6l 63 and 65, 31 and E9 are-supplied from rheostats 1|. 13 and,15, respectively, connected across auxiliary buses 11 and 13, 19 and M, and 31 and 11,

respectively, which derive-their power from intermediate taps 83 of the deltasecondaryof a transformer 85 supplied from the main'buses 21, 2-3 "and 25. The taps 83 are connected each through a resistor 81, SI and 93 toaneutraf bus'95.

' 39-to49 through the secondaries 91, 99,, fill, I03,

itself, however, both as to its organization and 7 its methodcf operation, together with additional objects and advantages thereof will be understood from thefollowing description of :a specific V embodiment when read in connection withthe accompanying drawings in, which Figures 1.2 and 3 together constitute a circuit dia am-of a preferred embodiment of our invention; 1

Fig. 41s a circuit diagram showing a modification of our .invention and, r 7

' Fig. 5 is .a graph illustrating the operation of the apparatus shown in Figures. 1 to 4.

The, apparatus shownirrEigs. '1 to 3, comprises awelding transformer. 1 across the secondary 9 ofwhichapair of seam welding rollers I I are con,- nected. Therol'lers, enga e the material I3 to be welded as it passes betweenthem. and conduct the welding current. "lfhev primary of the welding transformer 1 is made up of' a plurality of windi'ngs-I5, l1'andI9, V v V The transformer '1' derives its, power from the buses 2I, Hand of a polyphase, supply which is preferably of'the, commercial 60 cycle type but maybe of any othertype... Each of thewindin'gs I 5;, I1 and I3,is-connectedacrosstwo oithese'bus'es through a pair of ignitrons 21 and29, 3I and 33,

r and and 31, respectively, in anti-parallel. For firingeach ofthe, ignitrons Z'l-to a firing thyratron 39, f I, .473, 45,41. and 4-9;,respectivelyisprm vided. The .thyratrons 39-17039 are so controlled that during a. predetermined number of half periods of'' the supply (2%, 23, 25), current of'one polarity is conductedithrough each of the primaries I5, I1 and I9 in its turn'and during a sub,- sequent numberof half periodsof'lthe supply, cur 'rentfof the opposite polarityis conducted through each of the primaries in itsturn. ji'i'or controlling the firi-ngthyratronsr-39 to, we provide frequency determining and" firing control circuits 5] and 53, respectively, of such character that the duration andthemagnitude ofthe current of one polarity which flows through the welding transall) Firing potential is supplied to the 'thyratrons I05 and III! of firing transformers H19, III, H3, H5, H1 and H9, respectively; The primary I21 of the transformer I99 is supplied with current through'the thyratron 59- and isconnected between the anode I23 of this thyratron. and the movable tap I25 of the rheostat1I from which the thyratron is supplied. The cathode I21 of the thyratron 59 is connected to the neutral bus .95; The primary I29 of the transformer III is connected between the tap I25 and the cathode I3I of the thyratron 3|; the anode I33 of this thyratron is connected to the bus 95. The primaries I35 and I31 and i39 and I4I of the other pairs of firing transformers H3- and H 5 and. H1 and l I9, respectively, are similarly connected between the taps I43 and 1450f the rheostats13 and 15, respectively, and the anode MT of thyratron 61 and the cathode I53 of the thyratron 61. The cathodes I55 and I51" of thyratrons B3 and61, respectively, and the anodes I 59;and' I61, of thyratrons 65 and 69 are connected to the neutral 7 bus 95;

In the control circuits of one set of the firing control circuit thyratrons 53, '33, 611a common biasing network I33 is connected. In the control circuits of the other group of thyratrons 6|, 65, 59 separate biasing networks I65, I61 and IE9, respectively, are provided. These. networks I63, I55, I61 and I69. maintain thethyratrons .58, to 69 non-conductive.. 'Thethyratrons 61', 65 and 39 require separate networks, because their cathodeszlil, Ifl9and'l'53 are at the different potentials ofthe taps I25, Iji31andi I45, unlike I55 and" I51 of the other both thyratrons are non-conductive.

iiring transformer I09 in such mannerth'at' when the first thyratron 59 conducts the second thyratron 53, the conductivity of the second thyratron 63 is initiated approximately .9150) -o'f'a period of the supply after the first thyratron 59 begins to conduct. The output circuit'-of the second thyratron 63 is coupled through a secondary I19 of the associated firingj jtransformer II3 to the input circuit ofthe'. third thyratron 61 of the same group. The cou- "pling is such that when the second {thyration 63 is conductive, the third thyratron 61 conducts during an interval beginning of "a period after the thyratron 63 begins 'ito con- "duct. One thyratron SI of the other group (8|, 65, 69) is coupled to the secondary I8I of another output transformer I83 of the frequency determining circuit 5I and is rendered conductive by'potential derived therefrom. The other thyratrons 65 and 69 of the same group are coupled to the first thyratron 59 and (59, 53, 51) together in the same manner as the three thyratrons of the first group.

' 'The primaries I85 and I81 of the output transformers I13 and I83, respectively, of the frequency determining circuit 5I are each connected in the anode circuit of an output thyratron I93 and I95, respectively. The conductivity of these 'thyratrons I93 and I95 is so controlled that the output transformers I13 and I83 are supplied alternately during a predetermined number of half periods corresponding to the desired periodicity of the welding current. The welding current may have a frequency equal to any desired fraction of the supply frequency. However, to simplify the explanation we shall assume that the desired welding current frequency is of the order of 12 cycles per second. To provide current of this frequency each of the output thyratrons I93 and I95 of the frequency determining circuit 5I should conduct in its turn during two periods of the supply and should remain nonconductive during the intervening three periods. Between successive conductive intervals both .thyratrons are non-conductive during one period of the supply.

The conductivity of the thyratrons I93 and I95 of the frequency determining circuit is controlled from the weld interval timing circuit 55. The coupling of the timing circuit to the thyratrons of the frequency determining circuit is such that during the on intervals, each of the thyratrons of the frequency determining circuit conducts in its turn. During the oflf" interval, The "on" and olf intervals are timed by time constant networks I91 and I99, respectively. The on" network includes a capacitor I and a resistor 203; the ofi network includes a capacitor 205 and a plurality of resistors 201, 209 and 2 which may be selected in accordance with the desired .foff time interval. The on network I91 is connected in the control circuit 2]; of the on thyratron 2I5, and the "016 network I99 in the control circuit 2I1 of the oilifthyratron 2I9.

The on network I91 is also connected between a terminal of the secondary 22Iof. a transformer 223 and the cathode 225 of thefoff thyratron 2l9; the other terminal of the secondary MI is connected to the anode 221 cf this thyratron thyratron 2 I9 is non-conductive. While the oil thyratron 2I9 is conductive, the on capacitor 20I is charged to maintain the on thyratron non-conductive. The "on intervalis in the interval during which the on capacitor 20I discharges immediately following the instant when the oiT thyratron 2I9 is rendered non-conductive. During this on" interval, the on thyratron 2I5 is non-conductive. The off interval is the time interval during which the off capacitor 205 discharges following the instant when this interval, the off thyratron 2I9 is nonc'onductive.

Power is supplied to the start thyratron 23I and, the on thyratron 2I5 and the second output thyratron I from auxiliary buses 233 and 235 which are connected between a terminal tap and an intermediate tap of the secondary 231 of a transformer 239 supplied from the main buses -2I and 25. The conductivity of the start thyratron 23I is controlled by a switch 24I having a normally open contact 243 and a normally closed contact 245. When the switch is open, the normally open contact 243 maintains the anode circuit of the start thyratron 23I open between the bus 233 and the cathode 241. Initially, therefore, the OE capacitor 205 is discharged and the off thyratron 2I9 is conductive. At the normally closed contact 245 of the start switch a control circuit is closed through an auxiliary bus'249 connected to the other terminal of the secondary 231, a bias ng capacitor 25I in the control circuit 253 of the output thyratron I93, 3. grid resistor 255, the screen grid 251 of this thyratron, the cathode 258, to the bus 235. During the half periods when the bus 249 is positive relative to the bus 235, this capacitor 25I is so charged as to maintain the frequency determining thyratron I93 non-conductive.

The thyratron I93 is supplied through one section 259 of a doublediode 26| from the secondary 263 of another transformer 265. Between one terminal of this secondary 263 and the bus 235 (cathode 258) a time constant network 261 is connected. When the thyratron I93 is non-conductive the capacitor 269 of this network is discharged through its shunt resistor 21 I. The network 261 is connected between the control electrode 213 and the cathode 215 of an auxiliary thyratron 211. When the capacitor 269 is discharged this auxiliary thyratron 211 is conductive. The cathode 215 of the thyratron 211 is connected to the bus 235; the anode 219 is connected through a section 28I of. double diode 283 and a time constant network 285 to the bus 233. When the thyratron 211 conducts the capacitor 281 of the network 285 is charged. The network 285 is connected between the screen grid 289 and the cathode 29I of another auxiliary thyratron 293 and when the capacitor 281 is charged impresses a biasing potential on this thyratron. This bias may 'be' counteracted by potential impressed through a rectifier 295 on a capacitor 291, between the control grid 299 and the cathode 29I, from the secondary 30I of a control transformer 303. The primary 305 of this transformer is connected between the bus 233 and the anode 301 of the on thyratron 2| 5 through a section 309 of a double diode 3| I. r

4 Because the of! thyratron 2 I 9 is initially con 'ductive, the on capacitor 29I is charged and the 'on" thyratron 2I5 is initially non-conductive. With the on thyratron non-conductive potential is not suppliedto the control transformer 303 and the auxiliary thyratron 293 is nonmaintained non-conductive by the biasing capacitor 25I in spite of the fact that the capacitor =3I3 is discharged.

The anode 219 of the initially conductive auxiliary thyratron 211 is'also connected to the bus 233 through the other section 325 of the diode 1 183 andano'therltime constant network 321. Whenthyratron 211 conducts the capacitor 329 or this networlcisalso charged. The network 321 issconn'ected between the control electrode33I and the cathode 333 of the remaining frequency determining output thyratron 595 and maintains the latter non-conductive.

Initially then; potential isnot supplied from the output transformers I13 and I83 of the frequency determining :circuitSI and the thyratrons 59 to 65. of the firing control circuit are maintained non-conductive. The firing thyratrons 39 to 49 and theirassociated ignitrons 21 to 31 are, there- .fore, also non-conductive.

The operation of the seam welding system shown iniFigs. ilzto 3, isinitiated, after the electrodes II are engaged with the material-I3, by

load resistor 339., the thyratron193 to the 'bus Y 235. During each of these two positive half periods, a pulse is transmitted through the output-transformer I13 and through the grid'transformer 15 of the first thyratron 59 of one of the groups of thyratrons of'the firing control circuit.

scapacitor 34 I connected to the. secondary .343 of the transiormeriis charged through a rectifier 3 .35 in such a sense as to counteract th bias impressed on the thyratron and the thyratron 59 is rendered conductive.

21 or the firing transformer I03, the thyratron 59 to thene'utral bus 95. Through a secondary 310i thefiring transformer I93 firing potentialis impressed in the control circuit of the firing thyra't'ron39 for one of th ignitrons 21. The associated ignitron is rendered conductive and current :flowsfrom one of the three-phase buses 2| through the ignitron 21, one of the primaries I5 of the welding transformer 1 to another bus 23.

Through another secondary winding I11 of the firing transformer H19, a potential .is impressed through a rectifer 3'41 across a capacitor 349 in th control circuit 35I of the second thyratron B3 of the group through arectifier. This capacitor 349 is charged and the bias impressed on this second thyratron I53 is counteracted so that the thyratron is rendered conductive. Current now flows from the second adjustable tap I43 through Current is now conducted u from the'adjustable tap-25 through the primary theprimary I=35-of a second firing transformer :15

11.3.,- the second thyratron 63to theneutral bus 95.- This current flows with a phase delay of .approximately 1;50 following the initiation of current-flow through th first thyratron 59. Through a-secondary"winding ml of the second firing transformer H3 potential is induced in a control circuit of the second firing thyratron 43 torender the latter and'itsassociated ignitron 3| conductive. Current now flows from a bus 23 through the last-mentioned ignitron 3 I, an associatedtprimary-Il of the welding transformer 1 to another 1011 1.25. Through :the'other winding l19- of the second firing transformer H3 potential is pressed. in the control circuit 353 of the'third thyratron 310i the group to charge acapacitor 355 in. its control circuit and'render it conductive. Current theniflows from the third adjustabletap I45 through'the primary I3 of the third firing transformer I11, the thyratron '61 to the neutral 35. This cur-rent flows at a phase delay .of approximately 150 following the initiationof-current flow through the second thyratron 63. Through the secondary I of the third transformer potential is now'impressed to render the third firing thyratron 41 conductive andcurrent fiows through the associated ignitron 35 and through the third primary winding I3 of the welding transformer 1. r I

The above descriced sequence of opera-tions is initiated by the how of current through the thyratron of the frequency determining circuit during one half period'of the supply and takes wplace'dur ing'a full period of the supply. During the succeeding positive half period of the supply, theoutput thyratron 5'33 again conducts, a second potential pulsation is now again impressedin the'first 55 of the groups of thyratrons 59, 33,-31-"of the firing control circuit and the above-described operation is repeated, the ignitrons again being rendered conductive in succession and current "being supplied in succession to the primary windings [5, I1 and I9 respectively of the welding transformer 1. When the third ignitron 35 has conducted for the second time, six current pulses have been transmitted through the successive windings IE to it of the welding transformer and current ofsubstantial magnitude has been built up in the secondary E3 of the transformer.

While the output thyratron 193 is conductive, it'conducts current through a branch circuit extending from one terminal of the auxiliary sec- :ondary233, the other'section 25a of the diode '26 I, the thyratron I93, the time constant network .261 to the other terminal of the secondary. The capacitor 263 of this time constant network is chargedand the first auxiliary thyratron 211 is rendered nonconductive; The charging of the capacitors 281 and 323 ofthe twotime constant networks 285 and 321, respectively, connected'to thisauxiliary thyratron 211 is, therefore, interrupted. Both capacitors 23? and 323 now .discharge through their associated resistors351 and 359,v respectively. The resistor :95? of the network 285 is so selected that the network 285 reaches a potential at which thesecond auxiliary thyratron 293 is rendered. conductive approximately at the end of the second conductive half period ofthe first output thyratron I93 of the frequency-determining circu-it '51, and the resistor 3530f the other network 321 is so selected that the other outputrthyratron I35 is rendered conductive one period of the supply after this auxiliary thyratron 233Zis rendered conductive. When-thesecond auxiliary thyratron 293 conducts the capacitor 3I3 in its anode circuit is charged. The first output thyratron I93, therefore, fails to conduct during the first positive half period following the instant when the auxiliary thyratron 293 is rendered conductive; at this time the other output thyratron I95 is also non-conductive.

One full period after the end of the second conductive half period of the first thyratron I93, the second output thyratron I95 of the frequency determining circuit is rendered conductive. Current now flows through this thyratron and through the primary I95 of the second output transformer I13. Potential is now impressed in the control circuit 39I of the first thyratron BI of the second group BI, 65, 69 to render the latter conductive. Current is conducted through the primary I29 of the associated firing transformer III and firing potential is impressed in the control circuit 363 of the associated firin thyratron 4| firing the thyratron and its associated ignitron 29. Current now fiows from the bus 23 through the primary winding I5 of the welding transformer 1, the ignitron 29 to another bus 2I. This current is of a polarity opposite to the polarity of the first six current pulses. The firing transformer III is provided with another secondary winding 365 through which a firing potential is impressed in the control circuit 361 of the second thyratron 65 of the group and the latter is fired at a 150 phase delay after the first thyratron. An associated ignitron 33 is now fired and current is transmitted through another primary I1 of the welding transformer 1 in a direction opposite to the direction of the first six current pulses. The third thyratron 69 of the group GI, 65, 69 is rendered conductive from a secondary winding 398 of the second firing transformer H5 in its turn and an associated ignitron 31 is rendered conductive. Current then fiows through the third winding I9 of the transformer 1. This current is of the same polarity as the current flow through the other windings I5 and I1 just described. The second output thyratron I95 is now rendered conductive during a succeeding positive half period of the supply and the above sequence is again repeated so that six pulses of current of a polarity opposite to the polarity of the first six pulses is transmitted through the primaries I5, I1, I9 of the welding transformer 1.

While the second output thyratron I95 of the frequency determining circuit 5! is conducting, the capacitor 269 of the time constant network 261 in the anode circuit of the first thyratron I93 is discharging. After the second thyratron I95 has conducted during the positive half periods for two full cycles of the supply this capacitor discharges to a potential at which the first auxiliary thyratron 211 is rendered conductive. Current now flows through this thyratron to charge the capacitors 281 and 329 of the time constant networks in its anode circuit. The second output thyratron I95 of the frequency determining circult is now rendered non-conductive. The second auxiliary thyratron 293 is also immediately rendered non-conductive and the fiow of charging current to the capacitor H3 in the control circuit 323258 of the first output thyratron I93 is interrupted. This capacitor. is discharged through the resistor 3 I 1 and the secondary winding 3I9 of the heater transformer 32I. The discharge resistance is so selected that the first thyratron I93 becomes conductive one full cycle after the end of the second conductive half period of the second thyratron I95. The first thyratron now conducts during two full half periods while the second thyratron I95 is non-conductive, and

another six pulses of current of the first polarity are transmitted through the welding transformer 1. After the first thyratron I93 has conducted for two full periods, the second thyratron is again rendered conductive and the above-described sequence of operation is repeated. The first and second thyratrons thus continue to conduct and current of a frequency substantially lower than the supply frequency flows through the material I3 to be welded.

While the output thyratrons I93 and I95 or the frequency determining circuit 5I are conducting, the weld interval timing circuit 55 is also in operation. Current is conducted through the start thyratron 23I, the off capacitor 295 is charged and the off thyratron is immediately rendered non-conductive. The fiow of charging current to the on capacitor 29I is inter-.

rupted and the on capacitor discharges through the on resistor 293. This discharging operation continues for a predetermined time interval and so long as it continues the on thyratron 2I5 remains nonconductive. The frequency determining circuit 5| is not, for the time being, affected by the on thyratron.

When the on capacitor 29I has discharged for a predetermined time interval, it reaches a potential at which the on thyratron 2I5 may be rendered conductive. However, the screen grid 369 of the on thyratron 2I5 is connected to the time constant network 261 in the anode circuit of the first output thyratron I93 of the frequency determining circuit 5| and so long as" the first thyratron I93 is conductive, the on thyratron 2I5 is maintained non-conductive in spite of the fact that the on capacitor 29I is discharged. When the on capacitor ml is discharged and the first thyratron becomes nonconductive, the on thyratron 2 I 5 becomes 0011-. ductive after the capacitor 269 of the network 261 properly discharges through its associated resistor 21I. This latter event occurs when the first auxiliary thyratron 211 becomes conductive, that is just after the second output thyratron has conducted during two successive half periods of the supply completing a full cycle of the low frequency. Current now flows from the bus 233 through a time constant network 31I, the remaining section 313 of the double diode 3| I, a resistor 315, the on thyratron 2I5 to the bus 235. This time constant network 31I is connected between the control electrode 311 and the cathode 241 of the start thyratron 23I. The latter is immediately rendered non-conductive and the charging current to the off capacitor 295 is interrupted; the off capacitor discharges through one of its associated resistors 291, 299, 2I I. Until it is discharged to the proper potential, the off thyratron 2I8 remains non-conductive.

When the on thyratron 2I5 is conductive, current also fiows through the branch circuit including the primary 395 of a control trans-'- former 393. The second auxiliary thyratron 293 is, therefore, rendered conductive, in spite of the fact that the capacitor 281 of the network 285 is now charged, and the capacitor 3I3 in the control circuit of the first output thyratron I93 of the frequency determining circuit 5| is charged. The first thyratron I93 is now maintained nonconductive so long as the capacitor I I3 continues to be charged. Since this thyratron is non-con ductive, the auxiliary thyratron 211 remains con ductive and the second output thyratron I95 re- .tubes are conductive.

. r V 11 7 mains :I'lOn-COI'lduCtiVE. After the frequency determining circuit has completed its las-t'full cycle of operation and the last full cycle ofthe low-frequency has been transmitted through quiescent.

The above-described condition continues until the off capacitor 295 discharges through its associated resistor 291, 299, 2 to apotentialat which the off thyratron, 2l9 isrendered conductive. The on capacitor 291 is now charged and the on thyratron is immediately rendered non-conductive. Current fiow through the control transformer 3.93, theref lfl, stops and the first auxiliary thyratron 293 becomes non-conductive. Thereafter, the capacitor 313 in the control circuit 32:3- -2 58 of the first output thythe transformer I, the circuit 5| now remains r'atron I93 discharges'and this thyratron is rendered conductive initiating fa second welding operation. When the"on "thyratron 2I5 becomes non-conductive, the charging current to the capacitor 3'59 of the network 3' in thecontrolcircuit of the start thyratron 23! is interrupted, the latter, therefore, becomes conductive and the off capacitor 295 ischarged to render'the off thyratron 2l9 non-conductive. The second welding operation now in progress continues until theon thyratron 2 l5'again becomes conductive.

The operation of the seam welder shown in Figs. 1, 2 and 3, during an active interval is illustrated graphically by plots A, 'B and C of Fig. 5. In this figure, voltage is plotted vertically and timing horizontally. The shaded areas represent half periods or150 intervals during which Curve A illustrates the operation of the first frequ nc determining output thyratron I93, curve B that of-the second irequency determinin output thyratron 4'95 and curve C the operation of the associated ignitrons 2'l to 3?. The various shaded areas arelabelled Ignitron followed by a number or Thyratron followed by a number to correspond to the ignitron or thyratron which is conductive during the corresponding interval. l

The first output thyratron I93 is shown to be conductiveduring the first half period. Through the firing control circuit, the associated three ignitrons 21, 3| and 35 arerendered conductive in succession as shown. During .the succeeding positive half period the first thyratron [3918 again rendered conductive. and theassocia-ted isni rons a in conduct At the n of the sec-- ond full permamnowin th imtiat cn 1 he conductivity of he rs th rctrcn, c ch e he associated ignitrons have conductedtwo -cur t pulse a tctal of is cinc, condu ted th u h the t u fc mcr, h s h ra ron '9 new remains non-conductive for three I full periods (not including the first negative half perio'd fole lowing the last conductive positive haif period). The second frequency determining output thyrae tron I95 is now rendered conductive one full period after the instant when the conductivity of the first thyratron stops (as shown in cu ve B). During this period, the potential across the-phase conductors 2| and 23 between which current conducted initially during the first conductive V interval is at a maximum but of opposite polarity to the initial potential. The ignitron 2 connected in anti-parallel with the first 2] of the three ignitrons to conduct, therefore, becomes conductive. The other ignitrons '33 and '31 in the same group are conductive in succession following the conductivity of this'first-ignitron;

each of the otherignitrons 33 and 31' conduct-'- ing in the same order as the ignitron 3 and 35 =re-. spectively to which it is connected in anti-p'arah lel. The second thyratron l--also conducts during a second positive half period again rendering its associated ignitrons 2'9, 33 and '3'! each in its turn. After this conductive half period, the second thyratron is maintained non-conductive for three full periods during which-the first thyratron conducts. During the fullperiod during which both thyratrons F93 and 95 are non-conductive, the flux built up in the welding transformer by the current flow in one direction is permitted to decay. Approximately one half period is available for the decayto becompleted. In Fig. 4, a low frequency seam welding-system which derives its power from single phase buses 39] and 393 is shown. Inthis system, the welding transformer 395is of the type; having a primary 391 with an intermediate tap 399. This tap is connected to one o f the power supply-buses 39!. One of the terminals of the primary 391 is connected to the other bus 393 through a pair of ignitrons 40] and 403 in anti-parallel. The other terminal is also connected to the same bus 393 through another pair of; antiparallel ignitrons 495 and 491. Each of theignitrons 49L, 493, 49.5 and 401 is provided with a firing thyra tron 409, 4| l M3 and 415 respectively which is controlled from a firing. control circuit. The firms co clc c it s, upplied from a wc dilntc a i n rc and a. requ ncy tcrm m i cu t p ented F a 61 y a lock. 411 a d identical to that shown in Fig, l.

The s condary I' l o cncci he tput r nss f rmers 13 of the .fmcucncr, ete m ing ci cuit is con ec e hrou h a rect fier 9 acr s. a capac o 421 in the cont o ci cuit -23 o a thyra: ron 4.2 'of t fi ing contr l c c it- A- c,- c dar'y "5 of the o her control transfo mer 3 ss la y cted the cont ol circuit 427 f another t y a on 29. During the intervals w n ncy d ermining cir uit isacti- Y t d en l p l s are impressed du ng two successive positive half periods. of. the supply at intervals of threeperiods of the supplyalternatively in the. control circuits 423 and 421p: each of the thyratrons 42.5,and 429, respectively. h n e st pu e s i presse i he control ir u 423- of the firstt yra ron .425, the latter is renc lered conductive and current flows. from an auxiliary lineconductor-lltl through the Pri 33 a. fi ing ran rmer 435,1thc thy atr n! to cihcr, li 'c, on u to 31. The pcr ticnctthc cir uit 1 issc timed th t when thyratron 25 1.5. conduct vcbusz-ifii is p sitive and us 35'! ne ati e. Throu h. one Secondary .43 of the t ansfomcr 3.5 firmspcic 1 .7 impressed in. the ent a cir uita m of amine thyratron .499- and-the associated :ignitron Miis rendered conductive. Current 'fiows from-the now positive bus 393 through theignitrontlllgl downward through the upper half oiithe primary 397 of the welding transformer'395ito the :other bus 39L Through another. second-ary '442'a.potential is impressed in the control cireuit 443aof a second thyratron 445' torender-thelatter consv ductive during the succeeding half period 'ot the supply potential. This latter thyratron con ducts current from the bus 431" through the primary441 of a firing transformer 449, the thyratron to the other bus 43!. Through the secondary 45! of this firing'transformer 449, the firing thyratron M5 for; another 5 ignitron -49! is rendered-conductive. The ignitron nowwonatoms? ducts current from the now positive bus" 3% ofzthe supply, downward .through the lower-;,-ha1f .of the primary 391. of .the welding transformer, through the ignitron'401 to the remaining bus 393. During the succeeding half period of the supply, the first thyratron 425 is again rendered conductive firing .the associated ignitron 40! during one half period, and the second thyratron 445 is rendered conductive firing its associated ignitron 401 during the succeeding half period.

One full period after the instant when the current fiow through the first output transformer I13 stops the second output transformer I15 is energized. The thyratron 429 controlled from this transformer is now rendered conductive, conducting current through the primary 453 a firing transformer 455. Through one secondary 451 of this transformer potential is impressed to render the firing thyratron 4| I of an associated ignitron 403 conductive. Current now fiows from a now positive bus 39 l, upward through the upper half of the welding transformer primary 351, the ignitron 403 to the now negative bus 393. Potential is impressed to render a thyratron 459 associated with the last conductive thyratron 429 conductive through another secondary 4B! of the firing transformer. The latter thyratron is rendered conductive during the succeeding half period and its associated ignitron 405 is rendered conductive during this half period. Current now flows from the now positive bus 393, through the ignitron 405, upward through the lower half of the primary 391 of the welding transformer to the now negative bus 39!. The last-described process is repeated during another period of the supply when a second potential pulse is impressed through the second output transformer I15. Thereafter current is again conducted downward through the primary when ignitrons 40! and 401 are conductive. Current is thus supplied for seam welding during the selected welding intervals at a frequency equal approximately to 15 cycles per second. This frequency is substantially lower than commercial frequency and in many respects mentioned in the above specified applications has numerous advantages.

The operation of the system shown in Fig. 4 is illustrated in curves A, B, D and E of Fig. 5. Curve D illustrates the operation of one set of the valves and curve E the operation of the other set of valves. As shown, each of the first set of ignitrons 4M and 401 is conductive in its turn during the two periods during which the first frequency determining thyratron I93 is conductive. Thereafter, none of the ignitrons is conductive during a half period. During this interval, the flux in the welding transformer 395 decays. After this half period, the second frequency determining thyratron I95 becomes conductive and the other ignitrons 403 and 405 are rendered conductive to conduct current of the opposite polarity through the primaries of the welding transformer. Another non-conductive half period follows the last half period during which current is conducted through the primary 391 of the welding transformer 395.

While we have shown and described certain specific objects of our invention, we are fully aware that many modifications thereof are possible. Our invention, therefore, is not to be restricted except insofar as necessitated by the prior art and by the spirit of the appended claims.

We claim as our invention:

1. In combination an electric discharge device having a pair of principal electrodes and first and second control electrodes; a timer connected to said first control electrode for supplying thereto a first potential during predetermined on? time intervals and a second potential during predetermined off time intervals, and a second timer connected tosaid second control electrode for supplying to said second electrode,,duri-ng said on intervals, a third potential during predetermined first sub-time intervals and a fourth potential during predetermined second sub-time intervals.

2. In combination an electric discharge device having a pair of principal electrodes and first and second control electrodes; a timer connected to said first control electrode for supplying thereto a first potential during predetermined on time intervals and a second potential during predetermined off time intervals; a second timer connected to said second control electrode for supplying to said second electrode, during said "on intervals, a third potential during predetermined first sub-time intervals and a fourth potential during predetermined second sub-time intervals and connections between said second timer and said first timer for preventing the supply of said second potential to said first control electrode during said first sub-time intervals.

3. In combination, an electric discharge device having a pair of principal electrodes and first and second control electrodes; a timer connected to said first control electrode for supplying thereto a first potential during predetermined on time intervals and a second potential during predetermined off time intervals; said first timer including a second electric discharge device having a pair of principal electrodes and first and second control electrodes, said last-named principal electrodes being coupled to said first control electrode of said first-named discharge device to supply said first and second potentials thereto and said first control electrode of said second discharge device being connected to control the supply of said first and second potentials to said first control electrode of said first-named discharge device; a second timer connected to said second control electrode for supplying to said second electrode, during said on intervals, a third potential during predetermined first sub-time intervals and a fourth potential during predetermined second sub-time intervals, and connections betweensaid second timer and said second control electrode of said second discharge device for preventing the supply of said second potential to said first control electrode during said first sub-time intervals.

4. In combination, a first electric discharge device having a pair of principal electrodes and a control electrode; connections to said control electrode for controlling the conductivity of said discharge device; a first network including a first control component and a second electric discharge device having a pair of principal electrodes, connected in series with the principal electrodes of said first discharge device; a second network including a second control component and a third electric discharge device having a pair of principal electrodes, connected in parallel with said first network; a fourth electric discharge device having a pair of principal electrodes and a control electrode; connections between said last-named control electrode and said first control component for rendering said fourth 76 discharge device conductive when said first dis- 

